Generally, to manufacture a liquid crystal display, a liquid crystal cell containing liquid crystal, and a polarizing film are basically required, and an adhesive layer, or a pressure-sensitive adhesive layer for adhering the two should be used. In addition, a phase retardation film, a compensation film for wide view angle, a brightness enhancement film, etc. may be further adhered to the polarizing film to improve performance of the liquid crystal display.
Typically, a liquid crystal display comprises a uniformly aligned liquid crystal layer; a multi-layer polarizing film comprising an adhesive layer or a pressure-sensitive adhesive layer; a phase retardation film; and additional functional film layers.
The polarizing film comprises a uniformly aligned iodine compound or dichroic polarizing material. To protect these polarizing elements, such protection film as triacetyl cellulose (TAC), etc. is used to constitute multi layers. The polarizing film may further comprise a phase retardation film having an anisotropic molecular alignment, or a compensation film for wide view angle such as a liquid crystal film.
The aforementioned films are made of materials having different molecular structures and compositions, and so have different physical properties. Especially, under a high temperature and/or humidity condition, the dimensional stability according to shrinking or expanding of materials having anisotropic molecular alignment is insufficient. As a result, if the polarizing film is fixed by a pressure-sensitive adhesive, a shear stress caused by shrinking or expanding of the polarizing film under a high temperature and/or humidity condition remains, whereby light leakage phenomenon occurs at a region on which the stress is concentrated.
To improve the light leakage phenomenon, it is needed to reduce the shrinkage of the polarizing film at a high temperature and/or humidity condition. However, it is very difficult to remove the stress generated from a liquid crystal panel to which a polarizing film consisted of materials having different physical properties is attached. Another method to improve the light leakage phenomenon is to provide stress release function to a pressure-sensitive adhesive layer to fix the polarizer to a liquid crystal panel. Generally, rubbers, acryls, and silicones are commonly used as pressure-sensitive adhesive. Among these, acrylic pressure-sensitive adhesives are advantageous in the pressure-sensitive adhesive property, optical property, durability, and weatherability, and so have been most widely used for manufacturing pressure-sensitive adhesive composition for polarizer.
A general design to provide a pressure-sensitive adhesive layer with stress release function is to design the pressure-sensitive adhesive layer to have large creep property and easy deformability. A representative method thereof is to provide a pressure-sensitive adhesive composition with durability and stress release function under a high temperature and high humidity condition by mixing a low molecular weight polymer having or not having a small amount of crosslinking functional group with a high molecular weight polymer having crosslinking functional group to be able to react with multifunctional crosslinking agent.
For example, Korea Patent Laid-open Publication No. 1998-079266 intended to solve the light leakage phenomenon by providing stress release property to a pressure-sensitive adhesive composition, comprising 100 parts by weight of a high molecular weight acrylic copolymer having a weight-average molecular weight of 1,000,000 or more, 20-200 parts by weight of a low molecular weight acrylic copolymer having a weight-average molecular weight of 30,000 or less, and 0.005-5 parts by weight of a multifunctional crosslinking agent.
Japan Patent Laid-open Publication No. 2002-47468 intended to provide stress-release function to a pressure-sensitive adhesive composition for polarizing film comprising 100 parts by weight of a high molecular weight acrylic copolymer having a functional group having a weight-average molecular weight of 800,000 to 2,000,000; 5-50 parts by weight of a low molecular weight acrylic copolymer having a weight-average molecular weight of 50,000 or less, and not having functional groups (the degree of dispersion=1.0 to 2.5); a crosslinking agent; and a silane compound.
Also, Japan Patent Laid-open Publication No. 2003-49141 intended to solve the light leakage phenomenon by providing stress release function to a pressure-sensitive adhesive composition for polarizing film comprising a high molecular weight acrylic copolymer having a weight-average molecular weight ranging from 1,000,000 to 2,000,000 and having functional groups; a medium molecular weight acrylic copolymer having a weight-average molecular weight ranging from 30,000 to 300,000 and having less than two functional groups; a low molecular weight acrylic copolymer having a weight-average molecular weight ranging from 1,000 to 20,000 (the degree of dispersion=1.0-2.5) and having no functional groups; and a crosslinking agent.
The above references employ the technical feature to soften a final pressure-sensitive adhesive to improve stress release property of the pressure-sensitive adhesive. That is, the pressure-sensitive adhesive was designed to release local stress resulted from shrinking or expanding of polarizer by increasing the creep property and making the deformation easy against outside stress, as the modulus of pressure-sensitive adhesive is reduced by a low molecular weight material added.
But, recently, the monitor size becomes larger, such as 19-23 inches for the computer monitor, and 40 inches or more for liquid crystal TV. Thus, the need for low light leakage phenomenon has been increased more. Accordingly, to realize better low light leakage property than the prior art, the amount of low molecular weight material in the pressure sensitive adhesives should be remarkably increased. In this case, the durability may be reduced under high temperature and/or humidity condition. Also, due to the large amount of low molecular weight material, severe protrusion of adhesive may occur when cutting the polarizer, and so the operability of polarizer may be significantly decreased.
Thus, there have been needs to develop a new adhesive for polarizing film improving the light leakage phenomenon and preventing the operability decrease of polarizer by minimizing the reduction of modulus of final pressure-sensitive adhesive, without compromising major characteristics of polarizing film products such as durability and reliability in long term usage as well as under a high temperature and humidity condition, and a polarizing film using the same.